Microwave amplifier



Sept. 15, 1953 A. v. HOLLENBERG MICROWAVE AMPLIFIER 2 Sheets-Sheet 1 Filed Dec.

SePf- 15, 1953 A. v. HoLLENBERG MICROWAVE AMPLIFIER 2 Sheets-Sheet 2 Filed DBO. 11, 1948 /NVENTOR 5y A. M Hout-NBER@ 77 ,d ci@ A? 'MTQFNE/ Patented Sept. 15, 1953 MICROWAVE AMPLIFIER Arthur V. Hollenberg, Morris Plains, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application December 11, 1948, Serial No. 64,889

Claims. l

This invention relates to the subject-matter of the application of W. B. Hebenstreit and J. R. Pierce, Serial No. 38,928, led July l5, 1948. That application discloses amplifiers which utilize the interaction of a pair of closely coupled electron streams, having different average velocities, to secure gain.

One object of the invention is to secure increased gain over that obtainable in some of the embodiments of the Hebenstreit-Pierce invention.

A related object is to utilize more effectively the total electron current represented in the two interacting electron streams.

A further object is to allow the reduction of noise caused by the oscillation of groups of positive ions which may exist in the electron path by facilitating the removal of such ions.

According to a principal feature of the present invention, a pair of streams of charged particles (electrons, for example), which according to the I-Iebenstreit-Pierce invention are projected at different average velocities along a path of travel, are projected as tubular streams, one within the other. The outside transverse dimension of the inner stream is very nearly equal to the inside transverse dimension of the outer stream and the thicknesses of the stream walls are many times smaller than the respective stream transverse dimensions.

According to another feature of the invention, Aa high impedance rod may be inserted lengthwise of the inner beam and may be held at a potential suiciently diiierent from that of the vessel enclosing the streams to remove extraneous positive ions from the electron path. rThe tubular nature of the inner stream allows such Ia rod to be inserted without unduly disturbing the electromagnetic fields set up within the stream. If concentric tubular streams are employed, the rod can be situated along the common axis.

Additional objects and features of the present `invention will appear upon a study of the following detailed description of several speciiic embodiments. ln the drawings:

Figs. 1 and 2 illustrate two embodiments of the invention employing concentric tubular electron streams with a center ion removal rod; and

Fig. 3 shows details of the cathode structure of Figs. l and 2.

At least one embodiment of the above-noted Hebenstreit-Pierce application comprises an amplier utilizing a pair of concentric electron streams of circular cross-section, the inner of which is solid, to secure gain. It has since been shown that for a given flow of electron current,

a marked increase in gain may be had if the inner stream is also tubular in nature and approximates the outer stream in size. It has been found that the gain obtainable by interaction between two electron streams having diiferent velocities is a function of, among other things, the distance of separation between the two streams. Thus, a tubular inner beam will produce substantially greater gain than a solid inner beam having the same total current flow since more of the electrons comprising the tubular beam are thereby brought closer to those comprising the outer beam. Power dissipation and beam definition dimculties which would tend to appear if the total current fiow of a solid inner beam were increased in the hope of securing increased gain are avoided.

Referring particularly to Fig. 1, the elongated cylindrical tube envelope I@ is made of a highly conductive non-magnetic metal such as, for example, copper. The extreme left-hand end section of envelope Ill encloses a pair of axially aligned indirectly heated annular cathodes II and I2. The axes of cathodes II and i2 coincide with the central axis of envelope It! and cathode II is situated to the right of cathode 2. Cathode II has a larger diameter than cathode i2. Both cathodes II and l2 have electron-emissive coatings on portions of their right-hand surfaces, the coated portion of each cathode surface being circular in shape and having a width that is many times smaller than the cathode diameter. The outside diameter of the electron-emissive surface of cathode I2 is only slightly smaller than the inside diameter of the emissive surface of cathode I I, the difference being only a small fraction of either diameter and comparable to about half the width of either cathode. A more complete description of cathodes will be given in connection with Fig. 3.

A control grid I3 is situated to the right of cathode I 2 and a similar control grid I4 is located to the right of cathode It. Envelope I0 is provided with an inward-projecting iiange Iii to the right of grid I4 and an accelerating grid I5 is situated at that point.

Cathode i 2 is connected to the negative terminal of a main battery Il. An auxiliary battery i8 is connected between cathode I2 and grid i3, maintaining grid I3 positive with respect to cathode t2. Cathode II is connected to a point on battery Il that is positive with respect to cathode IZ, and an auxiliary battery le is connected between cathode Il and control grid lll, maintaining grid il positive with respect to cathode II. Metallic envelope it is connected to a point on battery l 7 that is positive with respect to both cathode Il and grid I 4. Grid IS, being connected to ange l5, is at the same potential as envelope I0.

A flat circular metallic collector plate 20 is situated at the right-hand end of tube envelope I0, is axially aligned with cathodes ii and i2, and is connected to the most positive point on battery i7. A glass seal 2l is provided at the extreme left end of envelope I e and a similar glass seal 22 is situated at the right-hand end, enabling the enclosure to be evacuated (by means not shown). A cylindrical focusing coil 2li, supplied with direct current from a battery or other suitable source (not shown) surrounds and is axially aligned with tube envelope i8 and provides a strong longitudinal magnetic iield within tube it.

The parts thus far enumerated comprise means for projecting a pair of concentric tubular electron beams 24 and 25 lengthwise of tube Iii with different average velocities. Due to the structure of cathodes li and l2 and the effect of the magnetic focusing held, the walls of beams 24 and 25 are thin with respect to the beam di` ameters and the outside diameter of inner beam 25 is very nearly equal to the inside diameter of outer beam 24. Due chiefly to this beam formation, practically all the electrons comprising outer beam 24 are in very close proximity to practically all the electrons comprising inner beam 25 and close coupling exists.

The potential difference between cathodes li and l2 and accelerating grid iii determine the electron velocities of beams 24 and 25, respectively. Since cathode I l is at a positive potential with respect to cathode l2, the average velocity of inner beam 25 is greater than that of outer beam 24.. Provided that certain quantities have the proper magnitudes, these quantities being principally velocity difference between the two streams and current density in the streams, electron streams 24 and 25 are capable of amplifying a signal that may be impressed upon either or both of them. The distance allowed for the travel of streams 24 and 25 should preferably be at least several wavelengths in the electron streams at the signal frequency; the greater the distance, the greater the amplification, up to an asymptotic limit.

The above-mentioned amplification has been described in detail in the previously noted I-Iebenstreit-Pierce application and will be described here only briey. Although the actual mechanism of energy interchange between closely coupled electron beams of diierent average velocities appears to be rather complicated and involved, amplification may be considered, :for purposes of explanation7 to take place as follows. A signal is somehow impressed upon one of the streams. A large number of methods and devices for so impressing a signal were disclosed in the Hebenstreit-Pierce application and one such device will be later described as adapted for use in the present amplier. The signal, as impressed upon the stream may, for example, take the form of electron density modulation. Through electron interaction, a signal corresponding to that impressed upon the rst stream appears in the second. The resulting disturbance on the second stream produces, through electron interaction, another disturbance on the rst stream, thus causing the initial disturbance to be reinforced. The increased disturbance on the first stream in turn causes reinforcement of the one appearing in the second stream. The reinforcing process continues as the electron streams progress along their allotted path of travel and the amplified signal may be removed at or near the end of the path.

As was noted above, in order to utilize the amplification powers inherent in a pair of such streams, suitable means must be provided for modulating one or both streams near their source by the wave to be amplied, and for abstracting the amplified wave energy from one or both streams near the end of their travel.

The example of such modulating means shown in Fig. l comprises a conductive wire helix 25 supported on ceramic rods '21 or on the inside surface of a ceramic cylinder. The helix 2S embraces b'oth streams 24 and 25 for a distance near the beginning of their travel path. An input lead 28 is brought in through a glass seal 29 and a hole 30 bored lengthwise in tube envelope Hl from its left end through to a larger diameter inner space beyond flange l5. Input lead 28 is coupled to a suitable point on helix 25 through a coupling condenser 3|, the tapping point on the helix and the size of condenser 3l beingr chosen to give an impedance match between the external input circuit and the helix. A suitable matching termination is provided at the righthand end of helix 26 by spraying a thin layer of conducting material on ceramic Vrods 2l at or near their right-hand e'nds. Helix is connected to tube envelope l0 and is at the same potential. Its inside diameter is somewhat greater than the outside diameter of outer stream 24.

For very small differences between the velocities of beams 24 and 25, the velocity of propagation of a wave along helix 26 may, for example, be made the mean between the beam velocities. For larger velocity separations, the velocity of one stream may be 'adjusted to equal the velocity of propagation along helix 2.

The means shown in Fig. 1 'for taking out the amplied signal is likewise a helix 32 surrounding electron beams 24 and 25 near the end of their travel, just short of collector 2). Helix 32 similarly connected to th'e external, in this case the output, circuit by an output lead S3. Output lead 33 is coupled to a suitable point on helix 32 through a coupling condenser 34, and is taken out through a hole 35 bored lengthwise in tube envelope I9 from its right end through to an inner space to the left of collector 20. Hole 35 is provided with a glass seal 36. As before, helix 32 is supported on ceramic rods 3l and a suitable matching termination is provided at the left hand end of helix 32 by spraying a thin layer of conducting material on the supporting rods 3? at or near their left-hand ends. As before, helix 32 is connected to tube envelope i9 and is at the same potential. The inside diameter of helix 32 is also slightly greater than the outside diameter of outer stream 24.

In Fig. 1, as in Fig. 2 to follow, the electrical rather than the mechanical aspects of the device have been stressed, and no attempt has been made to show how the various elements would be supported in relation to each other or how their assembly would be provided for, since it is thought that all such features can be supplied in an obvious manner from the mechanical side of the vacuum tube art. It is only pointed out here that suitable supports will be necessary in a practical embodiment for holding the parts in proper spaced relation to one another and for allowing for application to them of the necessary voltages. The tube can be evacuated in the usual manner. The diameter of tube envelope It may, if desired, be reduced somewhat over the portion which encloses that part of the path of travel of beams 24 and 25 lying between helices 2B and 32.

In the operation of devices of the character shown in Fig. l, it is found that a certain number or positive ions are present because of the practical impossibility of completely removing all extraneous gas from the tube. When positive ions exist in the path of the interacting electron streams 2li and 25, groups of them tend to oscillate at certain frequencies and produce noise It is, therefore, highly desirable to remove as many ions as possible from the path of the streams. For this purpose, a ceramic rod it is inserted along the axis of tube I0 extending from a point just to the left of cathode I2. to a point just to the left of collector 2G. Being of a ceramic material, the resistance of rod 38 is so high that for all practical purposes it may be considered to be an insulator. A very thin layer of conducting material such as, for example, colloidal graphite (aquadag) or finely divided metal is sprayed onto the surface of rod 38. The resistance of rod 38 is then still very high in comparison with a straight piece of wire of similar length, but is reduced from that of the unsprayed rod suiciently to make the total potential drop due to ion currents (which are generally of the order of microamperes) along the entire length of rod 38 small in comparison with the potential difference between rod 38 and envelope IB. The left-hand end of rod 38 is connected to a variable tap on battery I'I.

If the variable tap of battery Il is so adjusted that rod .te is held at a potential different from that of envelope It and helices 2t and 32, positive ions will tend to be removed. If rod 3S is positive with respect to envelope iii, it will tend to repel positive ions, driving them outward from the electron path to helices 2e and 32 or envelope II). If, on the other hand, rod 32 is negative with respect to envelope Ill, it will tend to attract ions, pulling them inward out of the electron path. The effectiveness of rod 33 in removing ions from the electron path is dependent to a large extent upon the difference in potential, positive or negative, between it and tube envelope Iii. However, since a large difference in potential between rod 33 and envelope Iii will tend to disturb streams .'lf'I and 25 by attracting or repelling them, as the case may be, it is necessary to arrive at a compromise. It has been found that rod 3d will, remove ions and that the tube will operate satisfactorily if rod 3B is maintained at a potential substantially equal to that of cathode I2 or at a potential positive with respect to envelope It by approximately the amount that envelope I2 is positive with respect to cathode lI.

The diameter of rod 38 is many times smaller than the inside diameter of inner electron stream 252 in order to avoid unduly disturbing the radio frequency field set up by the streams 2d and 25 to any appreciable extent.

In an experimenta-1 model of the device shown in 1 having a total length of about twelve inches, the following direct operating voltages were found to give satisfactory results: cathode i2, ground; grid I3, ten volts positive; cathode I I, twenty-uve volts positive; grid I4, thirtyfive volts positive; envelope I0, fty-ve volts positive; collector 2U, some potential greater than fifty-five volts positive; rod 38, either ground or eighty-five volts positive.

In Fig. 2, component parts of the device shown have been given reference numerals corresponding to similar parts shown in Fig. 1. The chief structural difference between the device shown in Fig. 1 and that shown in Fig. 2 is that the latter has a different type of signal abstracting means and a different type of ion removal rod.

Referring to Fig. 2, the inner conductor of a coaxial line 40 is connected to collector 2a and coaxial line 2li is taken out of the tube through glass seal 22. A short hollow metal cylinder lll surrounds and is axially aligned with collector 2E. Its right-hand end extends somewhat to the right of collector 2|] and is connected by a metal flange :i2 to the outer conductor of coaxial line dil. The left-hand end of cylinder .II extends somewhat to the left of collector 28 and holds a grid i3 between cathodes II and I2 and collector 22. The outer conductor of coaxial line 4i) is at the same potential as tube envelope IQ and the inner conductor is connected through a radio frequency choke 44 to the most positive point on battery Il. In practice, choke MI may be a quarter wave line.

As an alternative to the reduced diameter of tube envelope I0 between helices 25 and 32 in Fig. 1, Fig. 2 shows a hollow metal inner cylinder 45 which has an inside diameter corresponding to that of the reduced diameter section of envelope Iil in Fig. 1. Cylinder 45 is concentric with envelope IG, is held in place by a number of annular ribs /I connecting it to envelope It, and is situated in the space between helix 2t and grid e3. This arrangement and the one shown in Fig. 1 are examples of means providing such reduced diameters and may be used alternatively if a reduced diameter section is desired.

In Fig. 2, a ceramic rod tl is situated along the axis of tube Iii and extends from a point just to the left of cathode I2 to a point just to the left of grid 43. A fine wire helix is wound upon rod 4'! throughout its length, causing rod fil to present a high impedance to the radio frequency field set up within streams 2t and 5 and a low direct current resistance to ion currents. The low resistance to ion current ow renders the total potential drop along the entire length of rod ll small in comparison with the voltage of battery Il. The helix should be so wound upon rod 4l that the velocity of propagation of a wave along it is different from the velocity of propagation of the impressed signal along streams 2t and 25. The left-hand end of rod il is connected to a variable tap on battery Il. If it is desired to omit the helix from one or more sections of rod dl, some provision should be made to maintain the remaining sections having the helix at approximately the same potential.

The operation of the embodiment of the invention shown in Fig. 2 is much the same as that of Fig. 1. A signal is impressed upon one of two concentric tubular electron streams 2d and and is amplified due to energy interchanges between electrons or the two streams. 1n Fig. 3, energy is removed from streams 2a and by a gap i3-2B. Rod 4l operates to remove positive ions from the electron path in the manner as does rod 33 of Fig. l.

Although the diierent types of ion removal rods have been described only in connection with the respective embodiments of the invention in which they are shown, they are substantially interchangeable and may be used alternatively.

Details of the cathode structure of Figs. 1 and 2 are shown in Fig, 3. In Fig. 3, cathodes I l and i2 are axially aligned and are spaced along the common axis, cathode Il being shown situated to the right of cathode l2. Cathode Il comprises a short hollow metal cylinder which is circular in transverse cross-section. The inside diameter of the cylinder is of the same order of magnitude as the outside diameter and the cylinder contains an annular recess 553 for the purpose of accommodating a heating coil. Recess 5l) opens to the left in Fig. 3. A portion of the cylinder is of somewhat reduced inside diameter and extends slightly beyond the right-hand end of the rest of the cylinder. The right-hand surface 5l of that section is narrow with respect to the cathode diameters and is coated so as to cause it to emit electrons when heated. Cathode i2 is somewhat similarly constructed and in cludes a similar annular heating coil recess 52 which opens to the left. However, the outside diameter of cathode I2 is slightly less than the inside diameter of surface 5l of cathode Ii. A portion of the cylinder comprising cathode l2 extends somewhat beyond the right-hand end of the rest of the cylinder and has the same outside diameter as the rest of the cylinder. The right-hand face 53 of that section is coated similarly to surface 5i of cathode il and is also narrow with respect to the cathode diameters.

The following emitting surface dimensions for cathodes l! and l2 have been found to give satisfactory results and are given by way of example: outside diameter of surface 5|, .450 inch; inside diameter of surface 5|, .400 inch; outside diameter of surface 53, .370 inch; inside diameter of surface 53, .310 inch.

While the electron-emissive surface of each cathode has been shown as consisting of the area between two concentric circles, it is contemplated that areas of other shapes may be used if desired. For example, the curves dening the area need not be circular, but may take the form of ellipses or may even be irregular in nature,

Although the invention has been described largely with reference to several specific embodiments, various other embodiments will occur to those skilled in the art. In particular, many of the numerous devices for impressing a signal upon the electron streams and for extracting the amplified signal which were disclosed in the previously noted Hebenstreit-Pierce application may be used to advantage. The invention itself is to be limited only by the spirit and scope of the appended claims.

What is claimed is:

l. A. microwave amplifier which comprises an elongated enclosure defining a path of travel for charged particles, electrode means at one end of said path to direct a pair of closely coupled hollow cylindrical streams of charged particles, one stream within the other, lengthwise along said path at discretely different average velocities, whereby said streams of charged particles interact electromechanically with each other to produce gain as they progress along said path, electrode means at the other end of said path to collect substantially all of the charged particles of both of said streams, coupling means to supply a signal to be amplified to at least one of said streams, and coupling means to withdraw amplified signal energy from at least one of said streams.

2. An amplier in accordance with claim l in which said streams are essentially circular in cross-section and in which the difference between the average diameters of said streams is at least several times smaller than either diameter.

3. An amplier in accordance with claim 1 in which said streams are essentially circular in cross-section, in which the outside diameter of the inner stream is substantially the same as the inside diameter of the outer stream, and in which the thicknesses of the stream walls are at least several times smaller than the respective stream diameters.

4. An amplier in accordance with claim 1 in which both the difference between the average transverse dimensions of said streams and the thicknesses 0f the stream walls are at least several times smaller than either transverse dimension.

5. A microwave amplifier which comprises an elongated enclosure defining a path of travel for electrons, electrode means at one end of said path to direct a pair of closely coupled hollow cylindrical electron streams, one within the other, lengthwise along said path at dilerent average velocities, whereby said streams interact electromechanically with each other to produce gain as they progress along said path, electrode means at the other end of said path to collect substantially all of the electrons of both of said streams, coupling means operable at one region along said path to impress a signal to be amplified on at least one of said streams, and coupling means operable at another region along said path to extract amplified signal energy from at least one of said streams.

6. A microwave ampliiier which comprises an elongated enclosure defining a path of travel for electrons, electrode means at one end of said path to direct a pair of closely coupled hollow cylindrical electron streams in which the electromechanical coupling is substantial between substantially all of the electrons of one stream and substantially all of the electrons of the other lengthwise along said path at different average velocities, electrode means at the other end of said path to collect substantially all of the electrons of both of said streams, input coupling means at the upstream end of said path to impress a signal to be amplified on at least one of said streams, and output coupling means at the downstream end of said path to withdraw amplied signal energy from at least one of said streams.

'7. An electronic amplifying device in accordance with claim 6 in which the length of the portion of said path over which the said electromechanicalv coupling is substantial is at least several wavelengths in the electron streams.

8. An amplifying space discharge device which comprises an elongated conducting envelope defining a path of travel for electrons, charged particle projection means positioned at one end of said enclosure, charged particle collection means positioned at the other end of said enclosure, means including charged particle acceleration means positioned between said projection means and said collection means to direct a pair of hollow cylindrical streams of charged particles, one stream Within the other, at diierent average velocities along substantially the entire length of said path from said projection means to said collection means in an electromechanically interacting relationship with each other, a rod-like conducting element situated centrally within said streams and extending along said path, and circuit means coupled to said rod-like conducting element for maintaining it at a direct potential substantially different from said envelope.

`9. An amplifying space discharge device Which comprises an elongated conducting envelope deiining a path of travel for electrons, charged particle projection means positioned at one end of said enclosure, charged particle collection means positioned at the other end of said enclosure, means including charged particle acceleration means positioned between said projection means and said collection means to direct a pair of hollow cylindrical streams of charged particles, one stream within the other, at different average velocities along substantially the entire length of said path from said projection means to said collection means in an electromeehanically interacting relationship with each other, a rod-like conducting element situated centrally within said streams and extending along said path, said rodlike element having an impedance per unit length at the signal frequencies at least several times that of a uniform metallic conductor, circuit means coupled to said rod-like element for maintaining it at a direct potential substantially different rom said envelope, means operative at one region along said path for impressing an alternating signal on at least one of said streams, and means operative at another region along said path for extracting an alternating signal from at least one of said streams.

10. An ampliiier in accordance with claim 3 in which said streams are substantially circular in cross-section and are concentric, and in which said rod-like conducting element is elongated, disposed along the common axis of said streams, and has an over-all transverse dimension at least several times smaller than the diameter of either of said streams.

11. An amplier in accordance with claim 3 in which the over-all transverse dimension of said rod-like conducting element is at least several times smaller than the over-all transverse dimension of either of said streams.

12. A space discharge device comprising electron projection means and electron collection means spaced apart to dene a path of travel for electrons, means to direct a pair of substantially tubular electron streams, one within the other, along said path from said projection means to said collection means in an energy interchanging relationship, whereby said electron streams interact to produce gain as they progress along said path, a rod of insulating material situated within said streams and along said path, said rod having a transverse dimension at least several times smaller than the transverse dimension of either of said streams and having a thin layer of conducting material on its outer surface, whereby the resistance of said rod is at least several times that of a uniform metallic conductor, and means coupled to said rod for maintaining it at a potential substantially different from that of said path defining means.

13. A space discharge device comprising electron projection means and electron collection means spaced apart to denne a path of travel for electrons, means to direct a pair of substantially tubular electron streams, one within the other, along said path from said projection means to said collection means in an energy interchanging relationship, whereby said electron streams interact to produce gain as they progress along said path, a rod of insulating material situated within said streams and along said path, said rod having a transverse dimension at least several times smallei` than the transverse dimension of either of said streams and having a ne metallic wire wound around it over at least a portion of its length, and means coupled to said rod for maintaining it at a potential substantially different from that of said path defining means,

14. A space discharge device in accordance with claim 13 which includes means coupled to at least one of said streams for impressing an alternating signal upon it and means coupled to at least one of said streams for extracting an alternating signal from it, and in which the irnpedance at the signal frequency of said rod with said wire wrapped around it is at least several times that of a uniform metallic conductor.

15. An amplifying space discharge device comprising an evacuated metallic enclosure, electron projection means and electron collection means spaced apart Within said enclosure to denne a path of travel for electrons, means to direct a pair of substantially tubular electron streams, one within the other, along substantially the entire length of said path from said projection means to said collection means in an energy interchanging relationship, whereby said streams interact to produce gain as they progress along said path, means operable at one region along said path for impressing a signal on at least one of said streams, means operable at another region along said path for extracting a signal from at least one of said streams, a high impedance elongated rod-like element situated Within said streams and along said path, and means coupled to said rod-like element for maintaining it at a potential substantially diierent from that of said metallic enclosure, whereby an electric eld is established between said rod-like element and said metallic enclosure which tends to remove positively charged particles from the paths of said electron streams.

ARTHUR V. HOLLENBERG.

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